The present invention relates to an optical communication system and particularly to a collimator-pair optical device which is used in combination with optical fibers.
The demand for increasing the capacity of an optical fiber communication network has been intensified with the rapid and wide spread of the Internet in recent years. The development of WDM (wavelength division multiplexing) communication as means for increasing the network capacity has been promoted rapidly. In WDM communication, individual pieces of information are transmitted by light components slightly different in wavelength. It is therefore necessary to use an optical functional device good in wavelength selection characteristic such as an optical demultiplexer, an optical filter, an optical isolator or an optical circulator. It is a matter of course that the functional device is intensively demanded in terms of manufacturability, reduction in size, integration, and stability.
In most cases, the optical functional device is configured as follows. Light emitted from an end surface of an emission side optical fiber is converted into parallel luminous flux by a collimator. The parallel luminous flux is transmitted through a planar optical functional device having a function of a filter or isolator. Then, the parallel luminous flux is condensed by a condenser lens again, so that the condensed luminous flux is sent to an end surface of an incidence side optical fiber. A rod lens having a radially refractive index distribution, a spherical glass lens, or an aspherical molding lens, is used as each of the collimator and the condenser lens. The lens easiest to handle from the point of view of shape and aberration correction is a gradient index rod lens.
FIG. 1 is a schematic view showing an example of a collimator parallel pair using first and second plano-convex lenses 3 and 4 each made of a homogeneous material. Generally, in the parallel pair, two lenses equivalent to each other (lens thickness: Z) are disposed on opposite sides so as to be separated at a distance 2L from each other. In the case of lenses asymmetric in shapes of lens surfaces as shown in FIG. 1, the two lenses 3 and 4 are disposed in directions reverse to each other. That is, in the case shown in FIG. 1, the first lens 3 has a planar surface 30 as an incident surface, and a curved surface 130 as an exit surface. On the contrary, the second lens 4 has a curved surface 140 as an incident surface, and a planar surface 40 as an exit surface. The curved surfaces 130 and 140 may be spherical surfaces or may be aspherical surfaces. Optical fibers equal in mode field diameter to each other and having the same characteristic are used as an incidence side optical fiber 1 (hereinafter referred to as “light source fiber”) and an exit side optical fiber 2 (hereinafter referred to as “light-receiving fiber”). The distance WD between an end surface 10 of the light source fiber 1 and the incident surface 30 of the lens 3 and the distance WD between the exit surface 40 of the lens 4 and an end surface 20 of the light-receiving fiber 2 are made equal to each other to thereby form a completely symmetric optical system.
FIG. 1 is a view in which light rays 5 are shown. Luminous flux emitted from a single-mode optical fiber, however, can be regarded as a Gaussian beam as shown in FIG. 2. In this case, two lenses 3 and 4 need to be disposed so that a beam waist (BW) 26 of a Gaussian beam 7 is formed at a midpoint between the two lenses 3 and 4 in order to obtain good coupling efficiency of the collimator parallel pair. That is, a first beam waist 16 (with radius of w1) corresponding to light 17 emitted from the light source fiber 1 forms a second beam waist 26 (with a radius of w2) at the midpoint of the optical system and is coupled to the light-receiving fiber 2 in the position of a third beam waist 36 (with a radius of w3 equal to w1) by the second lens 4.
If the wavelength used, the NA (numerical aperture) of each optical fiber and the positions of the focal point and principal point of each lens are known, then the values of WD and L in the configuration of FIG. 2 can be designed by calculation based on so-called ABCD rules using elements of a light ray matrix. Theoretically, for example, detailed numerical formulae have been described in Foundation and Application of Optical Coupling System for Optical Device, Gendai Kougaku Sha (1991) written by Kenji Kawano. Some optical design software programs available on the market have such ABCD calculating functions.
However, the inter-lens distance, that is, the distance L between the lens 3 or 4 and the second BW 26 is not allowed to be selected to be larger than the maximum value Lmax because of the presence of the maximum value Lmax. The relation between WD and L in a lens with a focal length f is typically shown in FIG. 3.
In such an optical coupling system, the ratio of the power of light incident on the light-receiving fiber to the power of light emitted from the light source fiber, that is, coupling efficiency or coupling loss is an important characteristic parameter. If L is not larger than Lmax, coupling efficiency of 100% (coupling loss of 0 dB) can be obtained theoretically when the value of WD is selected suitably. On the contrary, if L exceeds Lmax, coupling loss increases rapidly (FIG. 4). Incidentally, the value of Lmax increases substantially in proportional to the square of the focal length of the lens.
Although a completely symmetric optical system has been described above as an example, the optical coupling system may be configured so that the light source is constituted not by an optical fiber but by a light-emitting device such as a semiconductor laser while the light-receiving unit is constituted not by an optical fiber but by a photo detector such as a photo diode. Also in this case, the system can be designed on the basis of application of a Gaussian beam as described above.
Results of the ABCD calculation are, however, only based on paraxial data. The ABCD calculation can hold upon the premise that each lens has no aberration and that there is no shading caused by the influence of shortage of the effective diameter of the lens. In a lens practically used in such an optical system, loss caused by various kinds of aberration residual in the lens is inevitably produced. For this reason, the inter-lens distance 2L and the coupling loss do not always have such a simple relation as shown in FIG. 4. It is further considered that the coupling loss changes when the condition of the focal length and aberration of the lens changes in accordance with the change of temperature and humidity. In addition, the change in volume and length of a component for holding the lens or optical fiber is one of causes of the coupling loss.